In electrographic imaging a latent image of electric charge is formed on a surface of a carrier. Toner particles that are attracted to the charge are applied to the surface of the carrier to render the latent image visible. The toned image is fixed, either by fusing the toner particles to the surface of the carrier, or by first transferring the toned image to a receptor and fusing, or otherwise permanently affixing, the particles to the receptor.
The latent image is produced by imagewise deposition of electrical charge onto the carrier surface. Typically, charged styli, arranged in linear arrays across the width of a moving dielectric surface, are used to create the latent image. Such processes are disclosed, for example, in Helmberger, U.S. Pat. No. 4,007,489; Doggett, U.S. Pat. No. 4,731,542; and St. John, U.S. Pat. No. 4,569,584.
After the latent image has been created on the surface of the carrier, toner is applied to produce a toned image. The toner is fixed to the surface of the carrier by heat fusing, or, if the toner is dispersed in a liquid, by drying and then, optionally, by heat fusing. When fixing is by drying only, the image is comparatively fragile and can be damaged by physical contact, abrasion, and the like.
Because color electrography needs only a carrier comprising a dielectric layer over a conductive support, it has, from its inception, been used for applications in which large size images are needed, such as for store graphics, posters, signs, banners, and other out-of-doors advertisements. In these applications the need for image protection is intense. Consequently, the color electrographic art has, from the outset, endeavored to protect the image.
One well-known way of protecting the image is to overlaminate the image with a permanent transparent protective film to form a laminate consisting of, in order, the carrier, the image, and the transparent protective film. In this method, the image is both fixed and protected between the original carrier and the transparent protective film in a single step.
In overlamination, the original carrier becomes an integral component of the final product. The carrier typically comprises a conductive base paper, which is unsuited for outdoor uses, even when protected by the overlaminated transparent protective film. Thus, overlamination is generally not suitable for preparing images for outdoor applications.
Another way of using the image, which avoids having the conductive base as part of the final product, is to laminate a receptor onto the toner that forms the image and then remove and discard the original carrier. In a second step, a permanent protective transparent film is laminated to the image and receptor, to form an element comprising the receptor, the toner image, and the permanent protective transparent film. The toner image is fixed and protected between the receptor and the permanent protective transparent film in two steps. This process is described in Chou, U.S. Pat. No. 5,262,259.
An alternative way that avoids having the conductive base as part of the final product is to laminate a receptor onto the surface of the dielectric layer which contains the toner that forms the image. Then the conductive base is removed and discarded, leaving behind the dielectric layer. This forms a product, comprising the receptor, the toner image, and the dielectric layer, in which the dielectric layer now serves as a protective layer. The image is fixed and protected between the dielectric layer and the receptor in a single step. Electrographic elements useful in this process are disclosed in Cahill, U.S. Pat. Nos. 5,414,502 and 5,483,321.
If the imaged and toned dielectric layer is to be transferred to provide a protective layer for the image, the dielectric layer must be able to release from the conductive base. To enhance release, a non-conductive release layer has been provided between the dielectric layer and the conductive base, increasing the cost and manufacturing complexity for the elements and lowering the density of the image. Therefore, to reduce manufacturing cost and improve image density, a need exists for electrographic imaging elements that do not comprise a separate release layer.